Compositions comprising mixtures of polycationic and polyanionic polymers as magnesium scale control agents

ABSTRACT

Mixtures of certain polycationic and polyanionic polymers derived from an ethylenically unsaturated dibasic acid or an ethylenically unsaturated sulfuric acid are effective compositions for the control of magnesium scale in evaporative desalination units.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to application Ser. No. 860,911 filed oneven date herewith. The instant application is directed to a mixture ofpolyanionic and polycationic polymers and the related case is directedto a process of inhibiting scale formation using a mixture ofpolyanionic and polycationic polymers.

This invention relates to a composition useful for reducing scalingassociated with evaporative desalination units. More particularly, thisinvention relates to such a composition comprising a combination ofcertain polycationic polymers and polyanionic polymers derived from anethylenically unsaturated dibasic acid and/or an ethylenicallyunsaturated sulfuric acid, the combination being soluble in water at uselevels without precipitation.

Desalination is a process for removing soluble salts from watercontaining them to render such water potable or useful in operationswhere the original salt content is unacceptable. Desalination is animportant process for providing potable water from sea water in aridareas where other sources of water are limited. Desalination is also animportant process for removing salts from waste waters so that suchwaters can be recycled to processing or safely discharged into naturalwaters. p Desalination may be an evaporative process carried out indesalination units that may be operated under vacuum, at atmosphericpressure, or at superatmospheric pressure. The use of vacuum orsuperatmospheric pressures are more difficult and costly to employ inconjunction with desalination units and, accordingly, the preferredprocedure of operation is essentially at atmospheric pressure. Inoperating such desalination units, the temperature of operation, whichis dependent upon the operating pressure employed, influences the natureof scaling that occurs. At atmospheric pressure, a transition pointoccurs at a temperature between about 80° C. and about 90° C., belowwhich the scale formation is due to calcium carbonate and above whichthe scale formation is due to magnesium hydroxide. While scale formationdue to calcium carbonate can be controlled by a number of usefuladditives, the problem of magnesium hydroxide scale or sludge control inevaporative desalination units has not been satisfactorily solved. Thus,although evaporative desalination units are potentially efficient in thedesalination of sea waters and waters of high salt contents, the rapidformation of magnesium scaling or sludging reduces efficiency,necessitating shut-down and scale or sludge removal from thedesalination units. The loss of operating time and the difficulties ofscale or sludge removal severely limit the amount of water processed bya desalination unit in a given time period and adds to the cost ofprocessed water.

A number of methods have previously been proposed for decreasingdeposition of scale from sea water onto metal surfaces and involve theuse of certain chelating agents. One such method involves addition ofpolyphosphate-lignosulfate mixtures, such as described in U.S. Pat. No.2,782,162 to Fidell. However, these mixtures are not effective in salinewaters at high temperatures and result in calcium phosphate sludge.

Also, certain polyelectrolytes, such as sodium polyacrylate, areeffective in preventing calcium carbonate scale at high temperatures,see South African Patent Application No. 680,947, but are ineffectiveagainst magnesium scales, as shown herein.

U.S. Pat. No. 3,981,779 to Block teaches use of a chelate surfactantselected from N-lauryliminodiacetic acid, N-oleyliminodiacetic acid,N-oleyliminodiacetic acid, oleoyliminodiacetic acid and their ammoniumand alkali metal salts. While these agents do decrease magnesiumhydroxide scale deposition to some extent, the extent of decrease ismarginal.

U.S. Pat. No. 3,985,671 to Clark, teaches use of a blend of apolyaminoalkylene phosphate and a polyquaternary ammonium chloride as ascale control agent for recirculating water cooling equipment. Thecombination is shown to be effective in extending the saturation pointof calcium carbonate, but no information is given as to itseffectiveness in preventing magnesium hydroxide scaling. However, suchcombination is found upon evaluation to be ineffective in preventingmagnesium hydroxide scaling to any desirable extent.

There exists, therefore, the need for a composition which is effectivein inhibiting the formation of magnesium scale or sludge in theoperation of evaporative desalination units. Such a provision wouldfulfill a long-felt need and result in a significiant advance in theart.

In accordance with the present invention, there is provided acomposition for inhibiting formation of magnesium scale or sludge inevaporative desalination units which comprises a mixture of (1) apolyanionic polymer containing at least about 50 mol percent ofrepeating units derived from an ethylenically unsaturated dibasic acidor an ethylenically unsaturated sulfonic acid and any balance ofrepeating units derived from one or more monomers compatible therewith,the acid units being in the form of at least one member selected fromthe group consisting of free acid radical, ammonium salt, and alkalimetal salts, and (2) a polycationic polymer selected from the groupconsisting of (a) dimethylamine-polyamine-epichlorohydrin reactionproduct wherein the amount of said polyamine is from 0 to about 15 molpercent of the total amine content and the amount of epichlorohydrin isfrom at least the molar equivalent of the total amine content up to thefull functional equivalent of said amine content, (b)poly(dimethyldiallylammonium chloride), (c) quaternarized derivatives ofpoly(dimethylaminoethylmethacrylate), and (d)poly[oxyethylene(dimethylimino)ethylene(dimethylinimo)ethylenedichloride], said polyanionic polymer having a molecular weight in therange of about 500 and about 50,000, said polycationic polymer having amolecular weight in the range of about 1,500 and 500,000, and the molarratio of said polycationic polymer to said polyanionic polymer based onthe average molecular weight of the repeating units therein being in therange of about 1.5:1 to about 25:1, respectively.

Use of the composition of the present invention in association withevaporative desalination units enables such units to be operated forgreatly extended time periods at high efficiency. This result is highlysurprising and completely unexpected in view of the fact that thepolyanionic polymer is ineffective in the inhibition of magnesium flocformation when used alone and the polycationic polymer is completelyineffective as an anti-scaling agent when used alone. In addition to itsability to inhibit magnesium hydroxide scale or sludge formation, theprocess of the present invention is also effective against calciumcarbonate scaling, thus providing protection against scale formation ata wide range of operating temperatures.

The polyanionic polymer useful in the composition of the presentinvention is a polymer containing at least about 50 mol percent ofrepeating units derived from an ethylenically unsaturated dibasic acidand/or an ethylenically unsaturated sulfonic acid. Ethylenicallyunsaturated dibasic acids are those that undergo free radicalpolymerization and include maleic and fumaric acids. Ethylenicallyunsaturated sulfonic acids also undergo free radical polymerization andinclude allylsulfonic acid. Any balance of repeating units of thepolyanionic polymer is derived from one or more monomers compatibletherewith. Preferred species include homopolymers of maleic acid andfumaric acid and copolymers of allyl sulfonic acid and either of theseacids. The acid units of the polyanionic polymer may be in the free acidform, ammonium salt form, or alkali metal salt form. The usefulpolyanionic polymers will have a molecular weight in the range of about500 to about 50,000. The polyanionic polymers in the form and molecularweight range specified must be water-soluble in useful concentrations soas to provide the level of effective dosage for use.

By the term "monomers compatible with" is meant those monomers whichwhen used to provide repeating units in the polyanionic polymer, provideunits in amounts which do not interfere with the function of the acidgroups present or adversely affect the solubility of the resultingpolymer. Suitable compatible monomers include acrylamide,methacrylamide, vinyl acetate, methyl vinyl ether, acrylonitrile and thelike. The polyanionic polymer may be prepared by conventionalpolymerization procedures using the selected monomer content or may beobtained by the hydrolysis of suitable polymers containing such startingmonomers as will provide the desired hydrolysis products followingconventional procedures. Polyanionic polymers useful in the compositionof the present invention are known in the art and, therefore, do notneed any further discussion herein.

The polycationic polymer may be any of four types. A first type isgenerally described in U.S. Pat. No. 3,738,945 issued to Panzer et al.on June 12, 1973, except that in the composition of the presentinvention the secondary amine is dimethylamine and the epoxy compound isepichlorohydrin or precursors thereof. Such a polycationic polymer isthe reaction product of dimethylamine, 0 to 15 mol percent of apolyfunctional amine based on the total amine content andepichlorohydrin, the amount of epichlorohydrin being from at leastequimolar to the total amine content to about the full functionalequivalency of the total amine content. The polymer should bewater-soluble and have a molecular weight in the range of about 1,500 to500,000, preferably about 10,000 to 200,000.

A second type of polycationic polymer is one consisting of repeatingunits of dimethyldiallylammonium chloride as disclosed in U.S. Pat. No.3,288,770, issued to Butler on Nov. 29, 1966. This polymer is referredto as poly(dimethyldiallylammonium chloride) and should have a molecularweight in the range of about 1,500 and about 500,000, preferably about10,000 to 500,000.

A third type of polycationic polymer is a quaternarized derivative ofpoly(dimethylaminoethylmethacrylate). This polymeric type is describedin U.S. Pat. No. 2,892,822 issued June 30, 1959 to Gray et al. Thesepolymers also should have molecular meights in the range of about 1,500and about 500,000, preferably about 10,000 to about 200,000.

A fourth type of polycationic polymer ispoly[oxyethylene(dimethylimino)ethylene(dimethylimino)ethylenedichloride]. This polymer in a limited range of viscosities is describedin U.S. Pat. No. 3,985,671 issued Oct. 12, 1976 to Clarke. The polymeris obtained by reacting dimethylamine and ethylene dichloride. Thispolymer will also have a molecular weight in the range of about 1,500and about 500,000, preferably about 3,000 to 50,000.

Both the polyanionic and polycationic polymers should be water-solublewhich in admixture are soluble at the concentrations at which they areto be used. Generally, these polymers are available at concentratedaqueous solutions which can be conveniently mixed in proper amounts toform the compositions of the present invention as concentrates which arethen readily diluted to the concentration of use. In preparingcompositions for use, the relative proportions of the two polymersemployed will be such that the molar ratio of the polycationic polymerto the polyanionic polymer based on the average molecular weight of therepeating units therein will be in the range of about 1.5:1 to about25:1, preferably about 1.5:1 to about 5:1. For example, in a polyanionicpolymer composed of 90 mol percent of sodium maleate units, MW=160, and10 mol percent of acrylamide units, MW=71, the average molecular weightof the repeating unit will be 151.1. Similarly, the repeating units of apolycationic polymer composed of dimethylamine and epichlorohydrin, willhave a molecular weight of 137.5.

In carrying out use of the composition of the present invention, themixed polymer composition is added to the water being processed in anevaporative desalination unit in an effective amount. By an effectiveamount is meant that amount which provides an increased operating timewithout significant formation of magnesium hydroxide scale or sludge.The specific amount of polymer mixture that will be effective in allinstances cannot be stated in a precise manner because the amount willvary widely due to a number of variables including the water beingprocessed, the conditions of operation of the desalination unit, thenature of the polymer mixture employed, and the like. An effectivedosage, however, will generally be found in the range of about 0.1 and100 parts polymer mixture per million parts of water. A preferred rangeis generally about 5 to 50 ppm.

The invention is more fully illustrated by the examples which followwherein all parts and percentages are by weight unless otherwisespecified.

Since the evaluation of scale inhibitors in commercial type desalinationunits involves large quantities of chemical additives, copiousquantities of process water, considerable expenditures of power toeffect vaporization, it is desirable to employ a small-size laboratoryscreening method whereby the suitability of processed additives can bepredicted with accuracy. The following laboratory method was employed inthe examples which follow and subsequent large scale evaluations oncommercial type equipment verified the accuracy of its predictions.

LABORATORY PROCEDURE

1. To a 150 milliliter (ml) capacity beaker containing 68 mls. ofdeionized water are added 5.4 mls. of 0.10 N sodium hydroxide.

2. To the solution obtained above are added 5 mls. of a 900 parts permillion (ppm) stock solution of the agent of test, giving 50 ppm on thefinal contents of the beaker.

3. After thoroughly mixing the resulting composition, 11.55 mls. ofmagnesium stock solution (6.0 grams Mg. (NO₃)₂ 6H₂ O in 1 liter ofwater) are added and heating to 90° C. with stirring is effected.

4. The beaker is then removed from the heat source and allowed to coolat ambient conditions.

A blank, no additive employed, run in this manner shows a white floc ofhydrated magnesia which settles out in about twenty minutes. Ineffectivescale inhibitors will have little or no effect on the rate of settlingof hydrated magnesia. Effective scale inhibitors show significantincrease in settling time of hydrated magnesia.

Following the Laboratory Procedure and using as the polyanionic polymera copolymer of 90 mole percent of acrylic acid and 10 mole percent ofacrylamide having a molecular weight of about 1,000, the followingpolycationic polymers were found to provide polymer mixtures ofineffective scale inhibiting properties at any molar ratio ofpolycationic polymer to polyanionic polymer:

Melamine-formaldehyde acid colloids

Mannich bases of polyacrylamide

Polyethylenimines

Cationic starches

Polyamines obtained by condensation of ammonia and ethylene dichloride

Polymeric reaction products of methylamine and epichlorohydrin.

Using the same polyanionic polymer, polymeric condensates ofalkylenediamines and dicarboxylic acid, a nonionic polymer, in admixturetherewith did not provide effective scale inhibition. Using as thepolycationic polymer, the reaction product of dimethylamine andepichlorohydrin and as the polyanionic polymer a polyalkylenephosphonatedescribed in U.S. Pat. No. 3,985,671, an ineffective scale inhibitor wasobtained. Thus, numerous combinations of polyanionic and polycationicpolymers are ineffective as scale inhibitors and it is surprising thatthe specific combinations of the present invention should be effectivescale inhibitors.

EXAMPLE 1

Using the Laboratory Procedure described above, the following run wasmade. The polyanionic polymer was a 50% aqueous solution of maleic acidhomopolymer of molecular weight 500 in the form of the sodium salt. Thepolycationic polymer was a 50% aqueous solution of the reaction productof an amine composition consisting of 98 mole percent of dimethylamineand 2 mole percent of ethylenediamine with an amount of epichlorohydrinequal to the full functionality of the two amines, the product having amolecular weight of about 10,000. The mole ratio of polycationic polymerto polyanionic polymer was 1.5:1, respectively, and was obtained byadding 4.0 parts of the polycationic polymer to 3.1 parts of thepolyanionic polymer. A free-flowing homogeneous composition dilutablewith water in all proportions was obtained. Evaluation of thiscomposition as described in the Laboratory Procedure resulted in theprevention of magnesium hydroxide floc for five days.

COMPARATIVE EXAMPLE A

An evaporative desalination unit operating at atmospheric pressure wasrun with sea water employing no additive to control scale formation.After approximately 24 hours of operation, heat transfer efficiency waslost due to magnesium scale formation and operation had to be stopped toremove the scale formation.

COMPARATIVE EXAMPLE B

Using a desalination unit as in Comparative Example A, a polycationicpolymer, which was the reaction product of a mixture of 98 mol percentof dimethylamine, 2 mol percent of ethylenediamine, and an amount ofepichlorohydrin equal to the full functional equivalency of the twoamines employed and of molecular weight 10,000 was added to the seawater being processed in the amount of 20 parts per million. Afterapproximately 19 hours of operation, heat transfer efficiency was lostdue to magnesium scale formation and operation had to be stopped toremove the scale formation.

COMPARATIVE EXAMPLE C

The procedure of Comparative Example B was followed except that insteadof the polycationic polymer employed therein, there was employed at 20ppm a polyanionic polymer which was a homopolymer of maleic acid in theform of the sodium salt and of molecular weight 500. Operation of thedesalination unit had to be stopped after about 48 hours due to pluggingwith magnesium hydroxide flocs.

EXAMPLE 2

The composition described in Example 1 is added to the sea water beingprocessin in a desalination unit as used in Comparative Example A in theamount of 20 ppm total polymer content. Heat transfer efficiency wasmaintained without the formation of scale or sludge for at least 110hours. Compared to the result obtained in Comparative Example A, thisrepresents an improvement in operating time of at least about 450%.

EXAMPLE 3

The procedure of Example 1 is repeated except that in place of thepolyanionic polymer used therein there was employed a copolymerconsisting of 50 mol percent of allyl sulfonic acid and 50 mol percentof maleic acid, the acid groups being in the form of the sodium saltsand the molecular weight being 1,500. The component polymers were mixedin proportions such as to provide a ratio of polycationic polymer topolyanionic polymer of 3.3:1, respectively, based on the averagemolecular weight of the repeating units. The composition was dilutablewith water in all proportions and provided similar results in scaleinhibition in the Laboratory Procedure to those obtained with thecomposition of Example 1.

EXAMPLE 4

The procedure of Example 1 was again followed except that in place ofthe polyanionic polymer used therein there was used a copolymer of 50mol percent of allylsulfonic acid and 50 mol percent of fumaric acid,the acid groups being in the form of the sodium salts and the molecularweight being 1,500. The component polymers were mixed in proportionssuch as to provide a ratio of polycationic polymer to polyanionicpolymer of 5:1, respectively, based on the average molecular weight ofthe repeating units. The composition was dilutable with water in allproportions and provided similar results in scale inhibition in theLaboratory Procedure to those obtained with the composition of Example1.

EXAMPLES 5-7

Again following the procedure of Example 1, a series of compositions areprepared and evaluated as scale inhibitors in the Laboratory Procedure.The compositions and ratios of components employed are given in Table Ibelow which also identifies the example number. In each instance, theresulting composition is dilutable with water in all proportions andprovides scale inhibition results in the Laboratory Procedure equivalentto that obtained with the composition of Example 1.

                                      TABLE I                                     __________________________________________________________________________         Polyanionic     Polycationic      Ratio                                  Example                                                                            Polymer (A)     Polymer (B)       B/A                                    __________________________________________________________________________    5    Homopolymer of Maleic Acid                                                                    Poly(dimethyldiallyl ammonium                                                                   2/1                                         Na Salt MW=500  chloride) MW=100,000                                     6    Homopolymer of Fumaric Acid,                                                                  Poly(trimethyolaminoethyl meth-                               Na Salt MW=500  acrylate methosulfate) MW=500,000                                                               3/1                                    7    Copolymer 50 mol percent                                                                      Poly[oxyethylene(dimethylimino)                                                                 4/1                                         Maleic acid and 50 mol per-                                                                   ethylene(dimethylimino)ethylene                               cent allylsulfonic acid,                                                                      dichloride]MW=3,400                                           Na Salt, MW=1500                                                         __________________________________________________________________________

I claim:
 1. A composition for inhibiting formation of magnesium salts orsludge in evaporative desalination units which comprises (1) apolyanionic polymer containing at least about 50 mol percent ofrepeating units derived from an ethylenically unsaturated dibasic acidor an ethylenically unsaturated sulfonic acid and any balance ofrepeating units derived from one or more monomers compatible therewith,the acid units being in the form of at least one member selected fromthe group consisting of free acid radical, ammonium salt, and alkalimetal salts, and (2) a polycationic polymer selected from the groupconsisting of (a) dimethylamine-polyamine-epichlorohydrin reactionproduct wherein the amount of said polyamine is from 0 to about 15 molpercent of the total amine content and the amount of epichlorohydrin isfrom at least the molar equivalent of the total amine content up to thefull functional equivalent of said amine content, (b)poly(dimethyldiallylammonium chloride), (c) quaternarized derivatives ofpoly(dimethylaminoethylmethacrylate), and (d)poly[oxyethylene(dimethylimino)ethylene(dimethylimino)ethylenedichloride], said polyanionic polymer having a molecular weight in therange of about 500 and about 50,000, said polycationic polymer having amolecular weight in the range of about 1,500 and 500,000, and the molarratio of said polycationic polymer to said polyanionic polymer based onthe average molecular weight of the repeating units therein being in therange of about 1.5:1 to about 25:1, respectively.
 2. The composition ofclaim 1 wherein said polyanionic polymer is a homopolymer of maleicacid.
 3. The composition of claim 1 wherein said polyanionic polymer isa homopolymer of fumaric acid.
 4. The composition of claim 1 whereinsaid polyanionic polymer is a copolymer of 50 mol percent of maleic acidand 50 mol percent of allyl sulfonic acid.
 5. The composition of claim 1wherein said cationic polymer is the reaction product of an aminecomposition consisting of 98 mol percent of dimethylamine and 2 molpercent of ethylenediamine with an amount of epichlorohydrin which isequal to full functionality of the two amines.
 6. The composition ofclaim 1 wherein said polycationic polymer ispoly(dimethyldiallylammonium chloride).
 7. The composition of claim 1wherein said cationic polymer is a quaternarized derivative ofpoly(dimethylaminoethylmethacrylate).
 8. The composition of claim 1wherein said polycationic polymer ispoly[oxyethylene(dimethylimino)ethylene(dimethylimino)ethylenedichloride].
 9. The composition of claim 1 wherein said polyanionicpolymer is a homopolymer of maleic acid and said polycationic polymer isthe reaction product of an amine composition, consisting of 98 molpercent of dimethylamine and 2 mol percent of ethylenediamine with anamount of epichlorohydrin which is equal to the full functionality oftwo two amines.
 10. The composition of claim 1 wherein said polyanionicpolymer is a homopolymer of fumaric acid and said polycationic polymeris the reaction product of an amine composition consisting of 98 molpercent of dimethylamine and 2 mol percent of ethylenediamine with anamount of epichlorohydrin which is equal to the full functionality ofthe two amines.